Field of the Invention
The invention relates to an electronic circuit, and more particularly, to a current control circuit for a driving circuitry system of a light-emitting diode (LED) component.
Description of the Related Art
Generally, an LED driving circuitry system is used to rectify an AC voltage from an electric grid for driving an LED component. A serious problem in this manner of driving is that the fluctuation occurring on the AC voltage from the electric grid induces fluctuation of the average current of the LED component.
Based on the principles of behavior of LEDs, only when an input voltage is higher than the forward turn-on voltage of the LED, the LED is turned on. Thus, the turn-on angle of the LED (that is the electrical angle which is converted from the time when the LED is turned on continuously in one period of the AC voltage) varies with the fluctuation occurring on the AC voltage from the electric grid, which results in fluctuation of the average current of the LED.
FIG. 1 shows the structure of a current LED driving circuitry system. In the LED driving circuitry system, an operational amplifier OP, a transistor M1, and a resistor RCS form a linear current control loop P1 to set the current IO flowing through the LED component. In the circuitry system, the LED average current IAVG is defined as the result obtained by dividing a product by the total time, wherein the product is obtained by multiplying the LED instantaneous current by the turn-on time. The average current IAVG is represented by Equation (1):IAVG=VREF′×ton/(RCS×T)  Equation (1):
VREF′ represents the voltage at the non-inverting input terminal of the operational amplifier OP and also represents the reference voltage of the current module composed of the operational amplifier OP and the transistor M1. VREF′ is provided by a voltage regulator, and it is always a constant. RCS represents the resistance value of the resistor RCS. ton represents the LED turn-on time (that is the sum of the turn-on times of the LED component in periods among the preset total time). T represents the preset total time. FIG. 2 shows the timing chart of the circuitry system in FIG. 1. According to the above equation and the timing chart of FIG. 2, when the voltage fluctuates in an increasing direction, the turn-on angle becomes larger (that is, ton becomes larger), and the LED average current increases. When the voltage fluctuates in a decreasing direction, the turn-on angle becomes less (that is, ton becomes less), and the LED average current decreases. The LED average current fluctuates with the fluctuation of the voltage from the electric grid, which induces LED flicker.